Chapter 5: Nitrogen Fixation and Respiration: Two Processes Linked by the Energetic Demands of Nitrogenase
Nitrogen fixation allows a diverse array of bacteria, either free-living in the environment or in symbiosis with plants, to grow in areas where fixed N is deficient. This confers them a large advantage over their non-N2 fixing competitors. Nevertheless, N2 fixation is an energy-demanding process, so that energy-generating respiration is oftentimes closely associated with efficient N2 fixing systems. The root nodule bacteria have many (O2-binding) terminal oxidases that differ in expression depending on whether the bacterium exists in free-living or in the symbiotic state. Many of the aerobic N2 fixers contain a very high O2 affinity terminal oxidase, like one that can function at free O2 levels as low as 7nM when the bacteria are surrounded in leghemoglobin within legume root nodules. The previously described ‘uncoupled’ nature of some oxidases of N2 fixing bacteria may not be the case, from more recent studies. The N2 fixing enzyme, nitrogenase is labile to O2 exposure; this creates a physiological problem for aerobic diazotrophs. The many strategies to overcome this problem must ensure a steady supply of ATP during nitrogenase function yet permit the enzyme to be protected from O2 inactivation. Strategies include living in aggregated or within viscous slimy sheaths, or in O2-restricted root nodule barriers or forming specialized cells lacking oxygenic O2-evolving photosynthetic machinery. The temporal separation of N2 fixation from O2 evolving photosynthesis is a mechanism used by some marine filamentous cyanobacteria. Employing vigorous respiratory activity at the membrane so that the inside of the cell is nearly anaerobic, or the use of a small redox-active iron-sulfur protein that binds to and protects nitrogenase from O2 inactivation are well studied mechanisms. Most of the protection mechanisms, even the expression of terminal oxidase activities are generally co-regulated with N2 fixation genes, and some of these protection systems may concomitantly combat oxidative stress. This additional stress is related to reactive oxygen species produced from N2 fixation related proteins.
KeywordsFree Living Terminal Oxidase Soybean Nodule Type Cytochrome SCHERICHIA COLI
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